1. Field of the Invention
The invention relates to a sensor with a diffusion conduit, for example, for measuring the exhaust gas of internal combustion engines, in accordance with the species of the main claim, in various embodiments, mostly as laminar or finger probes, which are embodied with various hollow chamber systems, i.e., measuring chambers.
2. Description of the Related Art
A sensor of the species is known from EP-A 0 188 900, wherein the design of the gas measuring chamber with its diffusion conduit is fixed by the relationship between distances which were empirically determined for different designs. This relationship was constant, independently of the design of the probe which was embodied flat, and determined the distances of the exhaust gas chamber in relation to the electrodes and their geometry.
The mathematical relationship m-1&gt;5 w, or in the borderline case m-1=5 w in EP-A 0 188 900, wherein 1 is the distance between the inlet of the gas measuring chamber and a next closest point of the first electrode in relation to the inlet, m is the distance between the inlet of the gas measuring chamber and a next closest point of the third electrode in relation to the inlet, and w the distance between the first and third electrodes exposed to the exhaust gas, relates to a planar arrangement which was laminarly constructed.
This formula loses its applicability in case of dimensions corresponding to atomic orders of magnitude, and in connection with very small dimensions would lead to the liquefaction of the gas because of wall reactions and adsorptions (Lin Zhang and Nigel A. Seaton, Prediction of the Effective Diffusity in Pore Networks Close to the Percolation Threshold, AIChE Journal 38, 1992, 1816-1824). Furthermore, because of the planar electrode arrangements, a measurement can mostly be performed only with homogeneous electrical fields.
A further limitation for gas flows from the gas measuring chamber in the disk-shaped diffusion conduit is the result of the right-angled deflection of diffused gas components, even in the case where w already assumes the multiple size of the diameter of the gas component.
Such a sensor element is constructed from a base body of, for example, zirconium dioxide ceramic material, which is used as a solid body electrolyte. For reasons of economics, a laminar construction of the sensor is advantageous, however, this is not the only design.
Examples of exhaust gas probes as EGOS (exhaust gas oxygen sensor), HEGOS (heated EGOS), PEGOS (proportional EGOS), UEGOS (universal EGOS) and TF-HEGOS (thin film HEGOS). Measurements are performed in the electro-chemical sensor element, among others in a marginal current probe, with at least two electrodes, one of which can come into contact with the reference gas, the other with the exhaust gas.
The gas component to be measured comes into complete contact with the porous electrode. This can lead to contamination or overloading of the contact surface in case of the appearance of saturation effects on the electrode surface because of the complete coverage with one or several gas components.
Sensors of this type operate in accordance with polarographic measuring principles. In the process, a constant electrode voltage is applied between an anode and a cathode and a diffused marginal flow is measured. However, the sensor could also operate in accordance with another electro-chemical measuring principle, for example, the potentiometric measuring principle.
The diffused marginal flow, for example in connection with a marginal flow probe, is defined by ions after they have overcome a diffusion barrier of the component of the gas to be measured, whose charges cause the current. The design of the gas measuring chamber, in particular the diffusion conduit ahead of the electrodes, determines the diffusion resistance of the measured gas and affects the gradient of the concentration of the measured gas component to be measured. A feedback to the control position of the sensor occurs.
Hereinafter, the term gas measuring chamber will also include the diffusion conduit and the electrode chamber, unless they are specifically mentioned. In a diffusion conduit which is a part of the gas measuring chamber of, for example, an exhaust gas probe, the gas mixture to be measured flows into the probe via the gas measuring chamber, which is fed from the outside. The gas measuring chamber should contain every gas chamber which can accommodate the gas component to be measured of the sensor.
The electrode chamber is the chamber located between the electrodes and containing the gas. It adjoins the diffusion conduit and at least the gas component to be measured flows through it.
Up to now, information and realizations which would permit the design of a gas measuring chamber for an exhaust gas probe which differs from a planar structure are greatly lacking. A disadvantage of the known sensors having a diffusion conduit tunnel is that the emission of signals remains sensitive to temperature and pressure or at least does not operate without interfering conditions.
A so-called mixed diffusion of Knudsen and gaseous phase diffusion exists in conventional embodiments with small dimensions of the diffusion conduit tunnel or with filled tunnels of the diffusion conduit, which could be the reason for the pressure dependency of the probe signals.
From DE-PS 37 28 289 it is known to embody diffusion conduits either with filler material for the Knudsen diffusion or without filler material for the gaseous phase diffusion and to design series and/or parallel circuits with the filler materials, which requires several production steps and results in spreading of the physical and chemical properties of the sensor specimen.